Coaxial cable is used to transmit radio frequency (RF) signals in various applications, such as connecting radio transmitters and receivers with their antennas, computer network connections, and distributing cable television signals. Coaxial cable typically comprises an inner conductor, an insulating layer surrounding the inner conductor, an outer conductor surrounding the insulating layer, and a protective jacket surrounding the outer conductor.
Each type of coaxial cable has a characteristic impedance which is the opposition to signal flow in the coaxial cable. The impedance of a coaxial cable depends on its dimensions and the materials used in its manufacture. For example, a coaxial cable can be tuned to a specific impedance by controlling the diameters of the inner and outer conductors and the dielectric constant of the insulating layer. All of the components of a coaxial system should have the same impedance in order to reduce internal reflections at connections between components. Such reflections increase signal loss and can result in the reflected signal reaching a receiver with a slight delay from the original.
Two sections of a coaxial cable in which it can be difficult to maintain a consistent impedance are the terminal sections on either end of the cable to which connectors are attached. For example, the attachment of some field-installable compression connectors requires the removal of a section of the insulating layer at the terminal end of the coaxial cable in order to insert a support structure of the compression connector between the inner conductor and the outer conductor. The support structure of the compression connector prevents the collapse of the outer conductor when the compression connector applies pressure to the outside of the outer conductor. Unfortunately, however, the dielectric constant of the support structure often differs from the dielectric constant of the insulating layer that the support structure replaces, which changes the impedance of the terminal ends of the coaxial cable. This change in the impedance at the terminal ends of the coaxial cable causes increased internal reflections, which results in increased signal loss.
Another difficulty with field-installable connectors, such as compression connectors or screw-together connectors, is maintaining acceptable levels of passive intermodulation (PIM). PIM in the terminal sections of a coaxial cable can result from nonlinear and insecure contact between surfaces of various components of the connector. A nonlinear contact between two or more of these surfaces can cause micro arcing or corona discharge between the surfaces, which can result in the creation of interfering RF signals. For example, some screw-together connectors are designed such that the contact force between the connector and the outer conductor is dependent on a continuing axial holding force of threaded components of the connector. Over time, the threaded components of the connector can inadvertently separate, thus resulting in nonlinear and insecure contact between the connector and the outer conductor.
Where the coaxial cable is employed on a cellular communications tower, for example, unacceptably high levels of PIM in terminal sections of the coaxial cable and resulting interfering RF signals can disrupt communication between sensitive receiver and transmitter equipment on the tower and lower powered cellular devices. Disrupted communication can result in dropped calls or severely limited data rates, for example, which can result in dissatisfied customers and customer churn.
Current attempts to solve these difficulties with field-installable connectors generally consist of employing a pre-fabricated jumper cable having a standard length and having factory-installed soldered or welded connectors on either end. These soldered or welded connectors generally exhibit stable impedance matching and PIM performance over a wider range of dynamic conditions than current field-installable connectors. These pre-fabricated jumper cables are inconvenient, however, in many applications.
For example, each particular cellular communication tower in a cellular network generally requires various custom lengths of coaxial cable, necessitating the selection of various standard-length jumper cables that is each generally longer than needed, resulting in wasted cable. Also, employing a longer length of cable than is needed results in increased insertion loss in the cable. Further, excessive cable length takes up more space on the tower. Moreover, it can be inconvenient for an installation technician to have several lengths of jumper cable on hand instead of a single roll of cable that can be cut to the needed length. Also, factory testing of factory-installed soldered or welded connectors for compliance with impedance matching and PIM standards often reveals a relatively high percentage of noncompliant connectors. This percentage of non-compliant, and therefore unusable, connectors can be as high as about ten percent of the connectors in some manufacturing situations. For all these reasons, employing factory-installed soldered or welded connectors on standard-length jumper cables to solve the above-noted difficulties with field-installable connectors is not an ideal solution.